Stick Together to Evolve

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Science  09 Sep 2011:
Vol. 333, Issue 6048, pp. 1360-1361
DOI: 10.1126/science.333.6048.1360-d

How did multicellularity arise? Koschwanez et al. propose that advantages in the use of limited amounts of food may have provided a selective advantage that promoted the evolution of single-celled organisms into multicellular forms. In yeast, simple mutations that alter the completion of cell division or prevent final digestion of the cell wall are sufficient to cause yeast to form aggregates or clumps of cells. Indeed, wild yeast grow in such multicellular forms, although common lab strains have been selected to grow as individual cells. Mathematical modeling and experiments show that when concentrations of sucrose are limiting, growing in clumps is advantageous because the cells secrete the enzyme invertase, which splits sucrose into the glucose and fructose the yeast use to fuel metabolism. Thus, it pays to be near other cells to have access to the sugars that are available in their immediate vicinity. In their experiments, clumps of yeast cells grew in concentrations of sucrose that were too low to support the growth of equivalent numbers of individual cells. Furthermore, clumping also helped cells compete with “cheater” yeast cells, which freeload off other cells and do not make any invertase themselves, thus gaining a small survival advantage. The occurrence of secreted enzymes pre-dates the emergence of multicellularity, so the authors propose that the social advantages such enzymes afford could have provided the impetus for the emergence of multicellular life forms.

PLoS Biol. 9, e1001122 (2011).

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